Technical Field
Embodiments of the invention relate generally to non-destructive evaluation (NDE) of wire ropes and cables during operation under load. Particular embodiments relate to measurements of distance along a loaded wire rope for purposes of NDE.
Discussion of Art
Many wire ropes for offshore applications, such as subsea construction ropes and mooring ropes, have large diameters (>100 mm) and lengths in excess of 2000 m. These ropes are expensive and frequently represent multimillion dollar investments, thus, they often are known as high value offshore ropes. For example, large mooring-lines for a wide variety of offshore structures can be classified as high value offshore ropes.
Other high value offshore ropes include winch-lines of pipeline-laying vessels. These ropes are utilized by offshore cranes for “Abandonment and Recovery” (A&R) operations of pipelines. A&R means to deposit a temporary pipeline termination on the seabed in order to take it up again later on. This process becomes necessary if temporary weather conditions do not allow a physical link between the vessel and the pipeline.
Especially the installation of subsea hardware requires extremely exact handling at high working loads. Operation on rough seas may result in the subsea hardware payload experiencing large oscillations, which can lead to instances of slack wire followed by large snap loads. Accordingly offshore cranes are equipped with so-called “Active Heave Compensation” (AHC) systems.
FIG. 1 shows an AHC system 170 that includes deflection sheaves 172, a storage winch 174, and a hydraulic piston 176 that thrusts and retracts a sheave 178 to accommodate slack and snap motions of a wire rope 12. Other types of AHC systems omit the hydraulic piston in favor of operating a storage winch in fast alternating cycles. An AHC system enables the offshore crane to compensate for heave motion so that the load connected to the crane hook can be lowered or lifted smoothly. Thus, heave compensation systems facilitate cranes, “Launch and Recovery Systems” (LARS) and other lifting equipment in operating in dynamic sea states. By installing a heave compensator, the dynamic payload motions can be mitigated and snap loads eliminated. Furthermore, a heave compensator enables the operator to keep the payload almost motionless with regard to the seabed or a fixed platform.
High-value ropes are safety critical. In contrast to smaller and less expensive ropes, they cannot be considered as disposable items. In spite of this situation, proper inspection methods for these ropes are only infrequently used. Most high value ropes are retired after predetermined service periods, irrespective of their actual condition. To protect the integrity of high-value ropes, the most sophisticated inspection and maintenance equipment and procedures available should be used. In particular, state-of-the-art magnetic rope NDE methods should be applied.
Realistically, a program of accurate and economically timed nondestructive inspections can extend the life of these ropes by several years, and possibly double their useful service life while, at the same time, maintaining safe operating conditions. Continuous monitoring will help with maintenance scheduling, so that advanced inspection and rope life evaluation methods can be used to plan rope retirement well in advance. For example, this would allow timely ordering a replacement rope.
A regime of regularly scheduled inspections will allow constant observation and data logging of the wire rope condition, which will help to establish maintenance schedules. This process will eliminate downtime for unexpected activities such as rope replacement, and it promises considerable savings by replacing the wire rope only when necessary and/or on planned maintenance schedules.
Regular and properly timed inspections can also serve as an effective preventive maintenance tool. To illustrate, here are some practical examples. Early detection of corrosion allows immediate corrective action through improved lubrication. Accelerating wear and inter strand nicking can indicate a need to reline sheaves to stop further degradation. Careful inspections can monitor the development of local damage at the crossover points of the rope on a winch drum. This way, the operator can determine an optimum time for repositioning the rope on the drum, in order to evenly distribute fatigue loads.
The following are examples of preventative operation and maintenance procedures that could be implemented by using a well designed rope monitoring program (RMP): For offshore cranes equipped with a heave compensation unit. The heave compensation system can quickly reduce the lifetime of the wire rope due to the large number of bending (fatigue) cycles over a short length of wire rope. An RMP could monitor the status of the rope and give a warning when its condition is no longer acceptable. Or an RMP could detect that a certain length of wire rope is almost worn out, and that the wire rope should be repositioned so that the almost worn out length is not subject to heave compensation. For conventional drilling rigs equipped with one hoist winch a ‘cut and slip’ practice can be used. Here, a large amount of spare wire rope can be stored on the drum. After a certain ton-mileage, the used section of wire rope is cut off, and a new unused wire rope section is slipped through the reeving. Other drilling rigs contain dual winch systems. Here, the travelling block is driven by two draw works at both ends of the wire rope. This is a fast, reliable and redundant drive system. By slowly spooling the wire rope from one drum to the other, the bend fatigue load is spread over the complete wire rope length.
Presently, it is a very conservative practice to replace the wire rope every year. However, an RMP could allow much longer intervals between rope replacements. Furthermore, if the condition of the wire rope is monitored by an RMP, its exchange can be planned in a timely fashion. An RMP will assess rope health and required safety margins on a continuous basis. This will allow optimum operation over the serviceable life of the rope. Another benefit of an RMP is to detect unexpected damage or corrosion. Then limits could be set within which all rope measurements must remain to ensure safe usage. Exceeding these limits would trigger an alarm that is distributed to responsible personnel for appropriate action.
Present length measurement systems measure distance incrementally and indirectly. For example, FIG. 2 shows schematically an incremental distance measurement system, which measures distance along a wire rope by counting the revolutions of an incremental rotary encoder 20 that is driven from a wire rope sheave (e.g., deflection sheave 178) or from a contact wheel 26. However, incremental measurements by distance counter wheels or sheaves are subject to systematic and cumulative errors, and they may not be repeatable. These deviations can be caused by a slightly oversized or undersized distance counter wheels or sheaves, slippage or other causes. The above incremental measurement method, using distance counter wheels, is presently the only known approach.